This invention relates to light gauging measurement methodologies and, in particular, to a method and apparatus for locating (reconstructing) the leading edge of an object such as an airfoil or the like. In aerodynamic structures such as airfoils, the majority of the performance characteristics of the structure are dictated by the characteristics (shape, contour, etc.) of the leading edge of the airfoil. Accordingly, in testing an airfoil to determine if it is manufactured to the proper specifications, accurately determining the location and shape of the leading edge is extremely important.
As shown in FIG. 1, an airfoil A has a leading edge E. A prior art test configuration for determining location of the leading edge employs a structured light sensor used to measure the position of the thin surface presented by the leading edge of the airfoil. In this setup, structured light from the sensor replaces the laser stripe typically used with a positional light source. This results in a linear highlight (the thin strip H) extending the length of the airfoil at its leading edge. The image of the edge highlight has properties similar to those of an image of a laser stripe over a smooth surface. This highlight can be processed using algorithms similar to those used to detect a laser stripe in a standard structured light test arrangement, so to locate points along the leading edge. One technique employed in this regard uses triangulation. Here, points on the edge are located by processing multiple images of the object taken using cameras located in different positions.
A problem with this approach is that if the cameras used to capture the images are placed indiscriminately, the triangulated positions will be biased. This is because when imaging a thin structure to determine an edge of it, various optical and imaging effects must be taken into consideration. For example, the angle of incidence of light from a source onto the surface of the object, occlusion boundaries, and illumination boundaries can produce an erroneous result so that the calculated location of the leading edge significantly differs from its true location. This can result in an acceptable airfoil being rejected, or an unacceptable airfoil passing. It is important therefore to provide an imaging system and test method which provides accurate leading edge location information.